The present invention relates to lenses for use in photography and related fields, particularly lenses of the type known as unsymmetrical or asymmetrical wide angle lenses. Many forms of such lenses are known in the art. The present invention deals particularly with improvements in lenses of the general type or configuration disclosed in the above mentioned U.S. Pat. No. 3,833,290 for an invention of the present applicants and another person. The entire disclosure of said U.S. Pat. No. 3,833,290 is incorporated herein by reference.
The various embodiments of the lens of the present invention have a number of characteristics in common with the lens of said patent. Among the characteristics common to the patent and to the present invention are the following:
1. The lens has at least five components air separated from each other and containing a total of at least eight separate lens elements. These components may also be referred to as members or groups, it being understood that the word "group" does not necessarily imply a combination of two or more lens elements, since some of the groups (if one chooses to call them groups) consist of only a single lens element. In the present specification they will be referred to usually as components, some of the components consisting of only a single lens element and some consisting of two or more lens elements closely associated with each other, possibly but not necessarily cemented to each other.
2. The above mentioned components comprise two components each of which, as a whole, has a condensing or converging action, these two components being separated from each other by a central vertex space in which a diaphragm of adjustable aperture may be placed, if desired. These two condensing components are identified, both in the patent and in the present application, as components C and D.
3. In front of these components C and D there are two meniscus components identified in the patent and in the present application as components A and B, both of which have front surfaces which are convex toward the front or longer conjugate or object side of the lens. These two components A and B have, in combination with each other, a dispersing or negative action.
4. The combined inherent refractive power .PHI..sub.AB of these two components A and B together has a negative value in the range from 1.19 to 0.24 times the equivalent refractive power of the entire lens system .PHI.. The combined refractive power .PHI..sub.AB of these two components is the reciprocal of their combined focal length f.sub.AB.
5. Either of these front components A and B may consist of a single lens element, or may be split into two lens elements. Both in said patent and in the present application, there are specific examples of lenses having split components and lenses having single element components in this location.
6. Behind the above mentioned central condensing components C and D, there is a negative component identified as component E both in the patent and in the present application, which component E has a rear surface which is convex toward the rear or image side of the lens.
7. The rear surfaces of the front meniscus components A and B are concave toward the rear, and the front surface of the rear negative component E is concave toward the front. Stated another way, the components A, B, and E all have concave surfaces toward the central vertex space, that is, the space between the components C and D, this space being identified as CS both in the patent and in the present application.
8. The front condensing component C is made up of at least two individual lens elements.
9. In this component C, the rear lens element (next to the central vertex space CS) preferably but not necessarily has an index of refraction which is lower than that of the forward element or elements of this component.
10. The second or rear one of the two condensing components, that is, component D, is made up of at least two individual lens elements, and includes a cemented surface which is convex toward the central vertex space CS, that is, toward the front of the lens.
11. In this component D, the front element of this cemented surface preferably has an index of refraction which is lower than that of the element at the rear side of the cemented surface.
As above indicated, all of the above mentioned features or characteristics are shared in common by at least some of the specific examples in said U.S. Pat. No. 3,833,290 and at least some of the specific examples disclosed in the present application. These features above mentioned indicate clearly the general type of lens with which the present invention deals, and serve as background for a better understanding of the improvement provided by the present invention, which is distinguished from the above mentioned patent by the further characteristics or features which will now be described.